Sexual selection may
facilitate speciation because it can cause rapid evolutionary diversification
of male mating signals and female preferences. Divergence in these traits can
then contribute to reproductive isolation. The sensory drive hypothesis
predicts

that three mechanisms underlie divergence in sexually
selected traits: (1) habitat_specific transmission of
male signals; (2) adaptation of female perceptual sensitivity to local
ecological conditions; and (3) matching of male signals to female perceptual
sensitivity. I test these mechanisms in threespine
sticklebacks (Gasterosteus spp.) that live in
different light environments. Here I show that female perceptual sensitivity to
red light varies with the extent of redshift in the light environment, and
contributes to divergent preferences. Male nuptial colour
varies with environment and is tuned to female perceptual sensitivity. The
extent of divergence among populations in both male signal colour
and female preference for red is correlated with the extent of reproductive
isolation in these recently diverged species. These results demonstrate that
divergent sexual selection generated by sensory drive contributes to
speciation.